1. Field of the Invention
The present invention relates to a frequency error detector and combiner in a mobile communication system.
2. Description of the Related Art
Typically, a mobile communication system is classified into a synchronous mobile communication system and an asynchronous mobile communication system. The asynchronous mobile communication system has been adopted in Europe, and the synchronous mobile communication system has been adopted in the U.S.A. Such a mobile communication system used in Europe is called a UMTS (Universal Mobile Telecommunication System), and a mobile communication terminal in the UMTS is called a UE (User Equipment).
In this system, a frequency offset is a factor causing unavoidable performance deterioration in a mobile communication system in that a carrier frequency gradually changes with temperature. Therefore, an AFC (Automatic Frequency Control) operation for compensating the frequency offset is needed. A reference signal of a frequency error control loop in the UMTS is a common pilot channel signal (hereinafter referred to as a CPICH).
FIG. 1 is a view illustrating a modulation pattern of the common pilot channel (CPICH). An average phase of the CPICH is calculated by selecting an arbitrary time period irrespective of a transfer rate of a traffic channel. The CPICH can calculate a phase variation from continuous CPICH signals in that it transmits non-modulated signals. That is, coordinates of a current reception symbol can be calculated by allowing a reception signal to be an I&D (Integrate & Dump)-processed during a predetermined period, and a phase variation of the current reception symbol is calculated by these coordinates and the coordinates of a previous reception symbol. Such a calculated value is a linear estimation value with respect to a low phase variation, and the linear estimation value is proportional to a frequency error. Such a phase variation occurs because a terminal has a relatively inaccurate timing with respect to a base station.
A timing reference of a terminal is a VCTCXO (Voltage-Controlled Temperature Crystal Oscillator). A timing error caused by a small frequency error of the VCTCXO occurs. Therefore, the timing error caused by a frequency difference between a terminal and a base station is corrected by controlling a control voltage of the VCTCXO. A phase variation of the CPICH detected by a phase error detector of an AFC module is multiplied by an appropriate gain in a loop filter and then indefinitely accumulated. An output signal of the loop filer is converted to a PDM (Pulse Duration Modulation) signal, and becomes a digital pulse signal for controlling a frequency of the VCTCXO.
The universal mobile telecommunication system (UMTS) supports an open loop transmission diversity and a closed loop transmission diversity as a transmission (Tx) diversity. In this case, the UMTS uses a STTD (Space Time Transmission Diversity) method as the open loop transmission diversity. In the case of supporting the STTD method, the UMTS transmits CPICH symbol patterns orthogonal to each other using two antennas. A frequency error detector detects frequency errors using a CPICH signal received from two antennas undergoing different independent fading phenomena, adjusts a gain by combining the frequency errors, and then transfers the resultant frequency error signal to a loop filter. In this case, the frequency error detector obtains a multipath diversity effect by combining time-delayed independent frequency errors generated by a multipath of a channel, thereby enhancing a performance of a control loop.
There are various kinds of method for detecting/combining frequency errors in a user equipment (UE) of the UMTS. Two of the methods are a,method using an arc tangent and method using a CPFDD (Cross Product Frequency Difference Detector), in the method for detecting frequency errors. The method using the arc tangent performs a normalization operation with a signal magnitude, but the other method using the CPFDD provides a frequency with a weighted value on the basis of a signal magnitude without performing a normalization operation. That is, since signal accuracy is degraded by noises in case of a low signal magnitude, the method using the CPFDD for providing a weighted value accurately estimates a phase error as compared with the method using the arc tangent in the actual channel circumstances. However, the method using the arc tangent estimates a phase error more accurately than the method using the CPFDD.
An algorithm using the arc tangent and another algorithm using the CPFDD in the UMTS system will be hereinafter described. The two algorithms use a previous value and a current value of the sum of two CPICH symbols as input signals to discriminate between antenna patterns. In other words, the algorithms perform an I&D process on received CPICH signals using 256 chips, remove an antenna pattern from the received CPICH signals, and have complex numbers shown in the following equation 1 on the basis of the sum of two symbols. Herein, CPICH signals express a complex number. The complex number comprises a real number and an imaginary number. R is the real number (or real part). I is the imaginary number (or imaginary part).CPICH1prev=R1+jI1, CPICH1current=R2+jI2CPICH2prev=R3+jI3, CPICH2current=R4+jI4  [Eq. 1]
A method for calculating a phase estimation value according to the arc tangent algorithm using the above complex numbers of a CPICH reference symbol can be expressed as the following equation 2, and a method for calculating a phase estimation value according to the CPFDD algorithm can be expressed as the following equation 3.
                                                                                          θ                  ^                                1                            =                                                tan                                      -                    1                                                  ⁡                                  (                                                                                                              R                          1                                                ⁢                                                  I                          2                                                                    -                                                                        R                          2                                                ⁢                                                  I                          1                                                                                                                                                              R                          1                                                ⁢                                                  R                          2                                                                    +                                                                        I                          1                                                ⁢                                                  I                          2                                                                                                      )                                                                                                                                          θ                  ^                                2                            =                                                tan                                      -                    1                                                  ⁡                                  (                                                                                                              R                          3                                                ⁢                                                  I                          4                                                                    -                                                                        R                          4                                                ⁢                                                  I                          3                                                                                                                                                              R                          3                                                ⁢                                                  R                          4                                                                    +                                                                        I                          3                                                ⁢                                                  I                          4                                                                                                      )                                                                                                                        θ                ^                            =                                                                    θ                    ^                                    1                                +                                                      θ                    ^                                    2                                                                                        [                  Eq          .                                          ⁢          2                ]            {circumflex over (θ)}1=Im{CPICHcurrent·CPICH*prev}=R1I2−R2I1{circumflex over (θ)}2=Im{CPICHcurrent·CPICH*prev}=R3I4−R4I3{circumflex over (θ)}={circumflex over (θ)}1+{circumflex over (θ)}2  [Eq. 3]
Phase estimation values calculated by the above equations 2 and 3 are combined according to a multipath.
However, provided that a first antenna 1 has a normal channel environment and a second antenna 2 forms a serious fading path if a frequency error is very low, a problem occurs even if the CPFDD is used. A first CPFDD 1 receiving a CPICH symbol of the first antenna 1 generates an output signal of almost zero, but a second CPFDD2 receiving a CPICH symbol of the second antenna 2 generates an inaccurate output signal because of serious fading. Complex planes of a CPICH reference symbol prior to performing such antenna diversity combination are shown in FIGS. 2a˜2b. It is anticipated that such a fading value is not high, but a magnitude of a residual frequency error can be increased according to a fading length (e.g, a fading duration). That is, a CPICH reference symbol from a first antenna 1 shown in FIG. 2a is not affected by noises of an oblique-lined area 10, but a CPICH reference symbol from a second antenna 2 shown in FIG. 2b is affected by noises of an oblique-lined area 10.